Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near K-edge spectra of SiO2 glass to 51 GPa obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below similar to 10 GPa are consistent with those of quartz and coesite, whereas the spectra above similar to 22 GPa are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in SiO2 glass between 10 GPa and 22 GPa. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed SiO2 glass.

Knowledge of the electronic structure of amorphous and liquid silica at high pressures is essential to understanding their complex properties ranging from silica melt in magma to silica glass in optics, electronics, and material science. Here we present oxygen near K-edge spectra of SiO2 glass to 51 GPa obtained using x-ray Raman scattering in a diamond-anvil cell. The x-ray Raman spectra below similar to 10 GPa are consistent with those of quartz and coesite, whereas the spectra above similar to 22 GPa are similar to that of stishovite. This pressure-induced spectral change indicates an electronic bonding transition occurring from a fourfold quartzlike to a sixfold stishovitelike configuration in SiO2 glass between 10 GPa and 22 GPa. In contrast to the irreversible densification, the electronic bonding transition is reversible upon decompression. The observed reversible bonding transition and irreversible densification call for a coherent understanding of the transformation mechanism in compressed SiO2 glass.

The strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe-57 isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mossbauer spectroscopy (nuclear forward scattering NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.

The strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe-57 isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mossbauer spectroscopy (nuclear forward scattering NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.

The multiferroic BiFeO3 crystals were studied under high pressures up to 70 GPa created in diamond anvil cells. Electronic, magnetic and structural transformations were found in the region 40-55 GPa. Electronic transition with high-spin (HS) to low-spin (LS) crossover at the Fe3+ ion, suppression of magnetism and metallization were carefully investigated and documented. All transitions are completely reversible. The proposed theoretical consideration explains some details of the high-pressure magnetic, optical and electronic properties.

The multiferroic BiFeO3 crystals were studied under high pressures up to 70 GPa created in diamond anvil cells. Electronic, magnetic and structural transformations were found in the region 40-55 GPa. Electronic transition with high-spin (HS) to low-spin (LS) crossover at the Fe3+ ion, suppression of magnetism and metallization were carefully investigated and documented. All transitions are completely reversible. The proposed theoretical consideration explains some details of the high-pressure magnetic, optical and electronic properties.

Nuclear resonant inelastic x-ray scattering spectra were measured for (57)Fe in disordered body-centered-cubic alloys of Fe-Cr. Partial phonon density of states (DOS) curves were obtained from these data. These results, in conjunction with the results of Ruckert [Hyperfine Interact. 126, 363 (2000)] on Fe-Cr thin-film multilayers and alloys, were analyzed with a local-order cluster expansion method. Interaction partial phonon DOS functions for the different short-range correlation functions were obtained from the disordered alloys. These interaction DOS functions were used in reconstructing the (57)Fe partial DOS curves measured by Ruckert on a set of thin-film multilayer samples of (57)Fe/(56)Fe/Cr. The method worked well using terms up to a combined first- and second-nearest-neighbor triangle cluster, which were obtained reliably from the disordered alloys. The limitations of a basis set of correlation functions from disordered alloys are discussed but shown to be acceptable for the chemical trends of phonons in the Fe-Cr system.

Nuclear resonant inelastic x-ray scattering spectra were measured for (57)Fe in disordered body-centered-cubic alloys of Fe-Cr. Partial phonon density of states (DOS) curves were obtained from these data. These results, in conjunction with the results of Ruckert [Hyperfine Interact. 126, 363 (2000)] on Fe-Cr thin-film multilayers and alloys, were analyzed with a local-order cluster expansion method. Interaction partial phonon DOS functions for the different short-range correlation functions were obtained from the disordered alloys. These interaction DOS functions were used in reconstructing the (57)Fe partial DOS curves measured by Ruckert on a set of thin-film multilayer samples of (57)Fe/(56)Fe/Cr. The method worked well using terms up to a combined first- and second-nearest-neighbor triangle cluster, which were obtained reliably from the disordered alloys. The limitations of a basis set of correlation functions from disordered alloys are discussed but shown to be acceptable for the chemical trends of phonons in the Fe-Cr system.

X-ray Raman scattering experiments were performed on C-60 fullerenes and multi-wall carbon nanotubes (MWCNT) up to 20 GPa and 25 GPa using a diamond anvil cell and synchrotron radiation at HPCAT, Advanced Photon Source. The intensity of the near edge peaks representing sp(2) hybridization (pi*) decreases with increasing pressure in both materials. Around 13 GPa the X-ray Raman spectra of C-60 completely transformed to sp(3) type bonding leading to diamond-like phase. Similar features have been observed in MWCNT at 16 GPa indicating the formation of strong interlayer covalent bonds. Our experiments are in excellent agreement with recent theoretical simulations reported on the mechanical behaviour of compressed nanotubes and provide direct experimental evidence for bond switching of these carbonaceous systems at high pressures. (c) 2006 Elsevier B.V. All rights reserved.

X-ray Raman scattering experiments were performed on C-60 fullerenes and multi-wall carbon nanotubes (MWCNT) up to 20 GPa and 25 GPa using a diamond anvil cell and synchrotron radiation at HPCAT, Advanced Photon Source. The intensity of the near edge peaks representing sp(2) hybridization (pi*) decreases with increasing pressure in both materials. Around 13 GPa the X-ray Raman spectra of C-60 completely transformed to sp(3) type bonding leading to diamond-like phase. Similar features have been observed in MWCNT at 16 GPa indicating the formation of strong interlayer covalent bonds. Our experiments are in excellent agreement with recent theoretical simulations reported on the mechanical behaviour of compressed nanotubes and provide direct experimental evidence for bond switching of these carbonaceous systems at high pressures. (c) 2006 Elsevier B.V. All rights reserved.